A light emitting diode (LED) is a semiconductor optical device capable of producing light in the infrared, visible or ultraviolet (UV) regions. LEDs emitting in the visible and ultraviolet are made using gallium nitride (GaN) and its alloys with indium nitride (InN) and aluminum nitride (AlN). These devices generally consist of p and n-type semiconductor layers arranged into a p-n junction. In a standard LED device, semiconductor layers are evenly grown onto a polished substrate such as GaAs or sapphire. A typical semiconductor layer is composed of gallium nitride (GaN) that has been doped to be a p-type or n-type layer.
Important figures of merit for an LED are its internal quantum efficiency (IQE) and light extraction efficiency, the product of which determines the external quantum efficiency (EQE). For a typical LED the IQE depends on many factors, such as the concentration of point defects, Auger processes and device design. In the case of nitride LEDs grown along polar (0001) and (000-1) directions the internal efficiency is also reduced by the distortion of the quantum wells between the n- and p-doped layers caused by the internal electric fields. The light extraction efficiency of standard LEDs based on GaN is determined from Snell's law to be 4% per surface. An LED commonly includes several quantum wells made of a small energy gap semiconductor (well) and a wider bandgap semiconductor (barrier). Visible LEDs employ indium gallium nitride (InGaN) as the well and GaN as the barrier. Ultraviolet LEDs employ AlGaN of different compositions as both wells and barriers. The IQE of an LED device based on nitride semiconductors grown along polar direction is reduced by electric fields across its quantum wells. This phenomenon is referred to as the quantum confined Stark effect (QCSE). The QCSE affects LED light emission by red shifting the emission wavelength and reducing its IQE. To mitigate the QCSE in GaN-based LEDs grown along a polar direction the quantum wells (QWs) are typically made very thin (less than 30 Å). In such thin QWs the electron and hole wavefunctions have sufficient overlap, which allows them to recombine radiatively. However, since such thin QWs consist of a few monolayers, there are variations in the thickness of the QWs across the wafer, which influences the yield and thus the cost of the current generation of LEDs. The rather small value of light extraction efficiency in the standard LED is the result of the high refraction index of the semiconductor layer at the exit interface.
A number of approaches have been proposed to enhance the extraction of light from LEDs. For example, in GaAs LEDs, the extraction of light is affected by the absorption of the emitted light in the GaAs substrate. To mitigate this problem, one can use epitaxial lift-off and wafer bonding methods to transfer the GaAs LED structure to transparent substrates. Another approach involving the optimization of LED surface geometry (such as the truncated inverted pyramid), combined with the use of substrate minors, has pushed the extraction limit to 30%. Other approaches involve the use of a continuously variable refraction index transparent material to reduce the back-reflection at the interface. Some of these approaches have manufacturing limitations and the last one suffers from fast index-material degradation with time.
An approach that is recently becoming increasingly attractive is photon extraction from randomly micro-textured thin film surfaces. It has significantly improved extraction efficiency, with record external quantum efficiencies of 44% demonstrated at room temperature for GaAs based LEDs (Windish et al., 2000). In this reference, the textured surface was formed after the growth of the LED using lithographic methods. It turns out that, even in that case, most of the photons are still extracted from within the emission cone inside the critical angle corresponding to a flat surface. More recently improved extraction efficiency in InGaN-GaN LEDs was obtained by roughening both the p-GaN surface and the back surface of n-GaN (Wei Chih Peng et al., Appl. Phys. Lett. Vol. 89, 041116 (2006)). The back surface was roughened after a laser lift-off from the sapphire substrate. The authors claim that the optical power of the LED increases by a factor of 2.77 compared with the conventional LED consisting of flat front and back surfaces. Consequently there is still wide room for improving light extraction well beyond the present values.
The advantage of III-nitride-based LEDs grown along nonpolar directions is that there is no internal electric field perpendicular to the QWs. As a result the QWs which constitute the active region of the device are not distorted, but maintain their square structure. In such square QWs the electron and hole wavefunctions have a strong overlap and thus they recombine radiatively. An obvious advantage of non-polar III-nitride LEDs is that the thickness of the QWs can be wide (of the order of 100 Å), which will lead to improvement in the yield and thus the cost of the LEDs. Non-polar LEDs are usually grown on the (10-10) plane, which is known as the M-plane or on the (11-20) plane, which is known as the A-plane. The advantages of non-polar III-nitride LEDs are discussed in an article by Waltereit et al. (Nature Vol. 406, 265 (2000)).
Another way to reduce the polarization effects in III-nitride LEDs is to grow the devices on semi-polar planes. Semi-polar planes are the (hkil) planes in which one of the h, k Miller indices is non-zero and the 1-index is also non-zero. Examples of semi-polar planes are the (10-1-3) and (10-1-1). In general, in III-nitride LEDs grown on semi-polar planes there will be a component of internal electric field perpendicular to the QWs, and thus the QCSE is not completely eliminated as it is in nonpolar LEDs. However, there are theoretical studies indicating that in particular semi-polar planes there will be zero net piezoelectric polarization in the growth direction (Takeuchi et al., Jpn. J. Appl. Phys. Vol. 39, 413 (2000)).
In the past few years the growth of nonpolar and semi-polar blue-green LEDs, whose active region is made of InGaN/GaN MQWs, has been demonstrated. For example, a semi-polar (10-1-3) InGaN/GaN green LED was deposited on a semi-polar (10-1-3) GaN template grown on M-plane sapphire by the HVPE method. In another example (Kim et al., Phys. Stat. Sol. Vol. 1, 125, (2007)) the investigators reported an efficient M-plane InGaN/GaN quantum well LED emitting at 402 nm. This device was grown on a free standing M-plane GaN substrate. The free standing M-plane GaN substrates were sliced from C-plane GaN bulk crystals grown by a hydride vapor phase epitaxy (HVPE) method. These substrates after slicing were mechanically and chemically polished prior to epitaxial growth. The sizes of these substrates was relatively small (3×25 mm) because of the relatively small thickness of the C-plane GaN ingots. An example of efficient blue LED grown on free standing semi-polar (10-1-1) bulk GaN substrates was reported by Zhong et al. (Appl. Phys. Lett. Vol. 90, 233504 (2007)). This device was also deposited on a small size (10-1-1) GaN substrate sliced from a C-GaN ingot grown by an HVPE method.
Growth of GaN on the R-plane of sapphire leads to A-plane GaN (Eddy et. al., J. Appl. Phys. Vol. 73, 448 (1993). It was shown that three unit cells of A-plane GaN fit in one unit cell of R-plane sapphire. Recently, it was also demonstrated that depending on the nucleation conditions on the R-plane sapphire one can obtain either the non-polar (11-20) A-plane or the semi-polar (11-26) AN and GaN films on R-plane sapphire (Chandrasekaran et al., Phys. Stat. Sol. (c) Vol. 4, 1689 (2007)).
The surface morphology of nonpolar GaN films grown on the R-plane of sapphire was found to depend on the conditions of growth. For example, Craven et al., (Appl. Phys. Lett., Vol. 81, 469 (2002)) have shown that nonpolar A-plane GaN films, grown by MOCVD on R-plane sapphire using the well known two-step growth process of a low temperature GaN buffer and a high temperature epitaxial film, have a specular surface morphology. Haskell et al., (Appl. Phys. Lett. Vol. 83, 1554 (2003)) reported the growth of nonpolar A-plane GaN films with specular surface morphology by HVPE without the use of a low temperature GaN buffer. These authors point out that their films are specular and the low-angle surface features scatter light minimally.
The state of the art of the IQE of the current generation polar InGaN-based LEDs is not well developed. However, the EQE has been reported to vary from 45% to 15% for LEDs emitting in the range from 370 nm to 525 nm (Harbers et al., J. Display Technology, Vol. 3, 98 (2007)).
Thus, there remains a need to improve the EQE of solid state optical III-nitride devices.